Hydreigon all the way.
lt has an AWESOME dual type, and has Levitate giving it an immunity to ground moves.

Haxorus can Dragon Dance all he wants, it's whether or not it can survive and take a few hits before it even gets to attack.
(lt's not super bulky!)
At least Hydreigon is fairly bulky.
Hydreigon doesn't even have that bad of Sp. Attack when compared to Haxorus' Attack, with close base Speed.
lt can also use it's high Sp. Attack to take advantage of Draco Meteor!

Why are you so attached to Haxorus? (I'm looking at bobathon, not Sam Sam Sam Sam) Even if Haxorus had a focus sash and used Dragon Dance, he won't always be faster on the first try. Hydreigon's speed is higher than Haxorus's. No offense, but Haxorus would have to be a higher level (by a lot) to pwn Hydreigon.

Haha looks like Unbreakable has never witnessed a well trained Haxorus at work in his party. 1 Dragon Dance is all it takes to out-speed a Hydreigon that isn't scarfed, even if both Pokemon are fully maxed out in speed EV's and IV's

Just my opinion here, but if you play your cards right while levelling, then Haxorus can retain his godly attack while have a decent amount of defence. Also, I tend to swap him in against anything that isn't dragon or ice, just call me paranoid.

So anyway, I gave mine a quick claw. This way, he might hit first every now and then. This, along with his swords dance makes him unstoppable.

Honestly, at level 64, EV training a good hydraigon is hard, because the first 2 evolutions, well, like all dragons, ARE WEAK AS. so honestly, in my opinion, HAXORUS.

I would definitely go with Haxorus.
First of all he has crazy attack power.
He will destroy you with outrage.
He will (at the right lvl) annihilate anyone (except ghost types) with guillotine.
And if you train him right he will have good defense.
one more thing, unless your willing to train a Siamese twin deino or something to lvl 64 than you should totally go with haxorus.